Electronic components on trenched substrates and method of forming same

ABSTRACT

A method of mounting an electronic component on a substrate includes forming at least one trench in a surface of the substrate. The trenches formed in the substrate reduce a stiffness of the substrate, which provides less resistance to shear. Accordingly, the trenches reduce the amount of strain on the joints, which mount the electronic component to the substrate, which enhances the life of the joints.

The present application is a Continuation application of U.S. patentapplication Ser. No. 12/030,274 filed on Feb. 13, 2008, which is aContinuation application of U.S. patent application Ser. No. 11/679,407filed on Feb. 27, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus forsurface-mounting capacitors on electronic (organic) substrates, and moreparticularly to a method and apparatus for reducing strain in asurface-mounted component.

2. Description of the Related Art

Electronic components, such as capacitors, are typically mounted on asubstrate. For example, as illustrated in FIG. 1A, a chip 102 is mountedon a surface of a substrate 104. A plurality of capacitors 106, or otherdiscrete electronic devices, is mounted on the surface of the substrate104, surrounding the chip 102.

FIG. 1B illustrates a detailed description of the electronic module 100,depicted in FIG. 1A.

Typically, the substrate 104 is formed by first forming the core 108. Anentire thickness of the substrate 104 may typically be, approximately 1mm thick. Next, build-up layers 110 are formed on each of a top surfaceand a bottom surface of the core 108. The build-up layers typicallyinclude polymer and copper layers.

A solder mask 112 is then formed on a surface of the substrate 104. Thesolder mask 112 covers the substrate, except areas where it is desiredto make a solder connection.

The chip (e.g., silicon chip) 102 is mounted to the substrate 104through solder joints 114. Similarly, the capacitor 106, or othersurface mount component, is mounted to the substrate by solder joints116 and pads 118.

Capacitors that are mounted on a substrate, in accordance with the abovedescription, however, may undergo significant thermomechanical strain.Temperature cycling produces fatigue of the solder joints or cracks inthe ceramic plates of the capacitor. Accordingly, the life of theelectronic package is reduced.

The electronic module is subjected to thermal cycling to evaluate therobustness of the electronic joints. FIG. 2 illustrates the sources ofstrain on a surface-mounted component during the thermal cycling.

First, shear stress 208 is induced in the solder joints 202, which mountthe capacitor 204 to the substrate 206. The shear stress on the solderjoints 202 is caused by a mismatch in coefficient of thermal expansionof the capacitor 204 and the substrate 206. Typically, the coefficientof thermal expansion for the capacitor 204 may be on the order ofapproximately 3 ppm, while the coefficient of thermal expansion of thesubstrate 206 may be on the order of approximately 20 ppm.

FIG. 3 illustrates the degree of stress 208 on the solder joints 202,the capacitor 204 and the substrate 206. The stress may also producecracks in the substrate 206.

FIG. 4 illustrates two types of strain induced on the solder jointsduring thermal cycling. Both Von Mises stress and shear strain areinduced on the solder joints.

Returning to FIG. 2, the mismatch in coefficient of thermal expansionalso causes the substrate 206 to bend. The bending of the substrate 206produces a tension/compression stress 210 on the solder joints 202.Additionally, while the bending may reduce the shear stress 208 on thesolder joints 202, it will cause an increase in the stress placed on thecapacitor 204.

Furthermore, the solder joints 202 have a coefficient of thermalexpansion, which may be different from the coefficient of thermalexpansion of the capacitor 204 and/or the substrate 206. This mismatchin coefficient of thermal expansion causes a localized, complex stressstate 212 on the solder joints 202.

Additionally, the stress at the solder joint areas permeates the body ofthe electronic components, which can produce cracks that may result infunctional failure.

Accordingly, prior to the present invention, there has been no method ofsurface mounting an electronic component, such as a capacitor, onto asubstrate while reducing the effects of stress on the joints, thecapacitor and the substrate.

SUMMARY OF THE INVENTION

In view of the foregoing and other exemplary problems, drawbacks, anddisadvantages of the conventional methods and structures, an exemplaryfeature of the present invention is to provide a method and structure inwhich strain is reduced in a surface-mounted electronic component.

In accordance with a first aspect of the present invention, a method ofmounting an electronic component on a substrate includes forming atleast one trench in a surface of the substrate.

In accordance with a second aspect of the present invention, anelectronic module includes a substrate having at least one trench formedin a surface of the substrate.

In accordance with a third aspect of the present invention, anelectronic substrate includes at least one trench in a surface of theelectronic substrate.

Forming trenches in a surface of the electronic substrate reduces thestiffness of the substrate in the vicinity of a capacitor (or any otherelectronic component), so that the substrate has less resistance toshear. Accordingly, the strain on the joints, which mount the electroniccomponent onto the substrate, is reduced, which enhances the life of thejoints.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other exemplary purposes, aspects and advantages willbe better understood from the following detailed description of anexemplary embodiment of the invention with reference to the drawings, inwhich:

FIGS. 1A and 1B illustrate an electronic module 100 including asurface-mounted capacitor 106 mounted according to a conventionalmounting technique;

FIG. 2 illustrates typical stresses induced in solder joints 202 of aconventional surface-mounted capacitor 204;

FIG. 3 further illustrates the stress induced on a conventionalsurface-mounted capacitor;

FIG. 4 further illustrates the stress induced on a conventionalsurface-mounted capacitor;

FIG. 5 illustrates an exemplary trenched substrate 502 in accordancewith an exemplary embodiment of the present invention;

FIG. 6 illustrates a first trench design in accordance with an exemplaryembodiment of the present invention;

FIG. 7 illustrates a three-dimensional view of the trench designdepicted in FIG. 6;

FIG. 8 graphically depicts the stress reduction resulting from thetrenched substrate of the present invention;

FIG. 9 illustrates a second trench design in accordance with anexemplary embodiment of the present invention; and

FIG. 10 illustrates a third trench design in accordance with anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 5-10,there are shown exemplary embodiments of the method and structuresaccording to the present invention.

FIG. 5 illustrates an exemplary electronic module 500 in accordance withan exemplary embodiment of the present invention. The electronic moduleincludes a substrate 502 having an electronic component (e.g.,capacitor) 506 mounted on a surface of the substrate 502. The capacitor506 is mounted on the substrate 502 through joints (e.g., solder joints)504.

In accordance with certain exemplary aspects of the present invention,at least one trench 508 is formed in a surface of the substrate 502. Byforming a trench 508 in a surface of the substrate 502, a stiffness ofthe substrate 502 is reduced. Reducing the stiffness of the substrate502 reduces the substrate's 502 resistance to shear forces. Accordingly,the stress induced on the substrate 502 and the solder joints 504 isreduced. Therefore, the life of the solder joints 504 is enhanced.

FIG. 6 illustrates a trench design in accordance with an exemplaryembodiment of the present invention. FIG. 7 illustrates athree-dimensional view of the electronic module illustrated in FIG. 6.

As shown in FIG. 6, an electronic module 600 includes an electroniccomponent (capacitor) 608 mounted on a substrate 602. The capacitor 608is mounted to the substrate 602 by a plurality of capacitor legs (solderpads/solder joints) 606.

In accordance with the exemplary embodiment depicted in FIG. 6, thecapacitor 608 is mounted to the substrate 602 by two rows of solder pads606. A continuous trench 604 is formed around each row of solder pads606. Each individual trench is approximately 15 μm deep and isapproximately 200 μm wide.

FIG. 8 is a graph depicting the reduction in stress achieved using thetrench design illustrated in FIGS. 6 and 7. As indicated in FIG. 8, thetrench design depicted in FIGS. 6 and 7 provides an 18% reductionstress.

FIG. 9 illustrates an alternative trench design in accordance withcertain exemplary embodiments of the present invention. The trenchdesign depicted in FIG. 9 also includes a continuous trench 902 formedaround each row of solder joints 904. Additionally, the trench 902includes trench extension slits 906 formed between each of the solderjoints 904. The trench extension slits 906 further reduce the stiffnessof the substrate, thus, further reducing the stress on the solder joints904.

FIG. 10 illustrates another alternative embodiment of the presentinvention. Similarly to the embodiments described above, the electronicmodule 1000 includes a capacitor 1004 mounted to a substrate 1002 by aplurality of capacitor legs 1006. In accordance with the presentembodiment, an individual trench 1008 is formed around each of theindividual capacitor legs 1006. The trenches 1008 have a shape similarto the shape of the capacitor legs. In the embodiment illustrated inFIG. 10, the trenches have a circular shape.

While the invention has been described in terms of several exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Further, it is noted that, Applicants' intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

1. An electronic module, comprising: a substrate having at least onestructure that reduces stress flow through the substrate, wherein saidstructure comprises at least one trench in a surface of the substrate.2. The electronic module in accordance with claim 1, wherein said atleast one trench is formed around a plurality of capacitor legs, saidcapacitor legs mounting the electronic component to the substrate. 3.The electronic module in accordance with claim 1, wherein said at leastone trench comprises: a first trench formed around a first row ofcapacitor legs; and a second trench formed around a second row ofcapacitor legs.
 4. The electronic module in accordance with claim 2,wherein said at least one trench comprises a continuous trench.
 5. Theelectronic module in accordance with claim 1, further comprising aplurality of trenches in said surface of said substrate.
 6. Theelectronic module in accordance with claim 5, wherein said plurality oftrenches comprises a plurality of continuous trenches formed around aplurality of capacitor legs.
 7. The electronic module in accordance withclaim 5, wherein said plurality of trenches comprises a plurality ofindividual trenches, said individual trenches being formed around aplurality of capacitor legs.